Golf balls are made in a variety of constructions and compositions. Golf balls, whether of solid or wound construction, generally include a core and at least a cover and/or outer coating. The core and/or cover can have multiple layers, such as a dual core having a solid center and an outer core layer, or a “dual cover” having an inner and outer cover layer.
Examples of golf ball materials range from rubber materials, such as balata, styrene butadiene, polybutadiene, or polyisoprene, to thermoplastic or thermoset resins such as ionomers, polyolefins, polyamides, polyesters, polyurethanes, polyureas and/or polyurethane/polyurea hybrids.
Typically, outer layers are formed about the spherical outer surface of an innermost golf ball layer via compression molding, casting, or injection molding. Cores are generally made using techniques such as compression or injection molding. For example, a center may be formed by compression molding a slug of uncured core material into a spherical structure. Meanwhile, outer core layers may be formed over the center by compression or injection molding techniques. In turn, the intermediate and/or cover layers are applied.
Suitable techniques for forming cover layer(s) over the core or intermediate layer (collectively referred to herein as “ball subassembly”) include, for example, compression-molding, flip-molding, injection-molding, retractable pin injection-molding, reaction injection-molding (RIM), liquid injection-molding, casting, spraying, powder-coating, vacuum-forming, flow-coating, dipping, spin-coating, and the like. In a compression molding process, hemispherical shells are generally placed about the subassembly in a compression mold and fused together under sufficient heat and pressure. In contrast, with an injection molding process, cover material is injected about and directly onto the subassembly using retractable pins, for example.
When a cover layer is formed by a casting process, liquid cover material is poured into lower and upper mold cavities, into which a subassembly is lowered at a controlled speed. The subassembly is held in place via partial vacuum to the point of sufficient gelling, and then the upper mold cavity is mated with the lower mold cavity under sufficient pressure and heat followed by cooling the unit until it can be handled without deformation.
And playing characteristics of golf balls, such as spin, feel, CoR and compression can be tailored by varying the properties of the golf ball materials and/or adding additional golf ball layers such as at least one intermediate layer disposed between the cover and the core. Intermediate layers can be of solid construction, and have also been formed of a tensioned elastomeric winding. The difference in play characteristics resulting from these different types of constructions can be quite significant.
Ionomers became popular golf ball cover materials due to their excellent impact resistance and their thermaplasticity, which permits the material to be economically applied via injection or compression molding techniques. Ionomers, particularly ethylene-based ionomers, are a desirable group of polymers for golf ball layers because of their toughness, durability, and wide range of hardness values. Further, golf balls incorporating fatty acid neutralized acid polymers are generally known for achieving desirable golf ball properties relating for example to spin, feel, and CoR.
The benefits and cost effectiveness of ionomeric/highly neutralized polymer (“HNP”) materials have therefore prompted some golf ball manufacturers to try producing golf balls with ionomers/HNPs in all layers. In this regard, U.S. Publ. No. 2006/0166759 of Kennedy III, et al. suggests incorporating a thermoplastic material such as an ionomeric composition or a highly neutralized blend “in at least one” of the core, cover or a boundary layer. Id. at ABSTRACT. Then, in U.S. Publ. No. 2006/0211518 of Sullivan et al., golf balls are disclosed having three or more adjacent layers wherein each layer contains an ionomeric/HNP material and a “percent neutralization gradient” either increases or decreases from innermost layer outward.
However, to date, golf ball manufacturers have not commercially pursued golf balls containing ionomeric/HNP compositions/materials in every layer—largely because resulting golf balls having sufficient resilience meanwhile have an undesirably hard feel. There is therefore a need for resilient golf balls containing ionomeric/HNP compositions/materials in every layer without the hard feel of prior golf balls. Golf balls of the present invention and the methods of making same address and solve this need.